The density of an image obtained in an electrophotographic image forming apparatus fluctuates if there is a change in the environment or a change in the components of the apparatus owing to prolonged use. In the case of a color image forming apparatus in particular, there is the danger that color balance may be upset even by a small fluctuation in density. This makes it necessary to maintain a constant density-tone characteristic at all times.
Accordingly, in an electrophotographic color image forming apparatus, means are provided for toner of each color, namely with correction means for correcting process conditions such as various amounts of exposure and developing bias conforming to absolute humidity, and tone correction means such as a look-up table (LUT). On the basis of absolute humidity measured by a temperature & humidity sensor, the process conditions and optimum value for tone correction are selected.
Further, a toner patch for sensing density is formed on an intermediate transfer body or drum using tone of each color so as to obtain a constant density-tone characteristic, the density of the unfixed toner patch is sensed by a density sensor for sensing the unfixed patch, and density is controlled by applying feedback to the process conditions such as amount of exposure and developing bias based upon the result of sensing, whereby a stabilized image is obtained.
However, density control using a sensor for sensing unfixed toner is such that a toner patch is sensed upon being formed on an intermediate transfer medium or drum. No control is exercised with regard to a change in color balance of the image ascribable to subsequent transfer and fixing to a transfer medium. In general, color balance varies also depending upon the transfer efficiency with which a toner image is transferred to a transfer medium and the temperature and pressure applied by fixing. Such a variation cannot be dealt with by density control that employs the aforementioned density sensor for sensing an unfixed patch.
An image forming apparatus proposed heretofore (for example, see the specification of Japanese Patent Application Laid-Open No. 2003-76077) is provided with a density or chromaticity sensor (referred to as a “color sensor” below) for sensing the density of a single-color toner image or chromaticity of a full-color image on a transfer medium following transfer and fixing, forms a color toner patch (referred to simply as a “patch” below) for controlling density or chromaticity, and feeds the sensed density or chromaticity back to the process conditions such as the amount of exposure and look-up table (LUT), thereby controlling density or chromaticity of the final output image formed on the transfer medium.
Further, in an ink-jet printer, color balance varies depending upon a temporal change in amount of ink discharged, a difference in environment and individual differences among ink cartridges, and therefore the density-tone characteristic cannot be held constant. Accordingly, in a printer proposed heretofore, a color sensor is placed in the vicinity of the printer output section, the density or chromaticity of a patch on a printing medium is sensed and density or chromaticity is controlled.
RGB data representing an image to be output in an image forming apparatus is converted by a color matching table to a device RGB signal handled by each type of image forming apparatus, the device RGB signal is converted by a color separation table to a YMCK signal that conforms to the toner or ink color, and the YMCK signal is converted by a calibration table to a Y′M′C′K′ signal that has undergone a density-tone characteristic correction specific to each image forming apparatus. Several methods of controlling density or chromaticity are conceivable. For example, an image of a desired tint can be obtained by correcting the calibration table based upon measured density or by correcting the color matching table or color separation table based upon measured chromaticity.
However, in a case where an image is fixed to each transfer medium using a multiple transfer media of different gloss and paper type in a conventional color image forming apparatus after application of “color stabilization control”, which includes forming a patch on a transfer medium, sensing the density or chromaticity of the patch and controlling density or chromaticity of the final output image based upon the result of sensing, whereby an image of stabilized tint is obtained, a problem which arises is that the tint of the toner of the fixed image differs depending upon the transfer medium. This problem becomes pronounced particularly at highlight portions of an image where the amount of toner build-up on the transfer medium is small.
Further, a color image forming apparatus such as a laser printer or copier is equipped with a plurality of paper-feed trays, a port for manual insertion of paper and paper-feed options. Since transfer media of multiple types usually are handled, this is an environment readily susceptible to the above problem. In other words, carrying out color stabilization control every time with the user being aware of the type of transfer medium is difficult in a case where use is made of a single image forming apparatus via a network. This subjects the user to a great burden. Consequently, such control is not realistic.
Furthermore, owing to an increase in the frequency with which color stabilization control is performed, intervals during which printing cannot be executed increase in length and there is also an increase in transfer media on which patches are printed.